With the advent of solid state control logic means such as microcomputers or microprocessors, a whole new field of control devices has evolved. When these devices are used in condition control or process control applications, the solid state control logic means or microcomputer ultimately controls electrical switching equipment, such as relays or heavy duty solid state switches. The ability of the microprocessor or microcomputer to carry out a large number of control functions in an exceedingly short period of time makes this type of device an ideal tool for monitoring and control of associated equipment. While the microcomputer or microprocessor operations entail possible failure modes that must be guarded against, they also provide an almost unlimited ability to monitor and control related equipment in fail safe manners not previously available in the control art.
In order to provide a degree of safety that is comparable with electromechanical types of devices, microprocessor or microcomputer type condition control systems must be operated with control routines that are significantly different than the mode of control applied to electromechanical of units. These routines form types of safety checking modes for the device. Specific safety checking circuits that dynamically check the switching circuits have been previously disclosed. These circuits rely on the control logic means or microcomputer and related circuitry external to the microcomputer for verifying dynamically that the load switching circuits have properly operated. These prior disclosed circuits do not check for safe operation of the microcomputer or control logic means itself. If a control logic means or microcomputer fails to operate in a proper manner, the external safety checking circuits are only of limited value. The control logic means itself must be rigidly checked for failure modes and for the transmission of signals in the form of "noise". When the load control circuits are checked by the microcomputer, and the microcomputer checks itself for proper operation, a new level of safety operation is made available.